Radioactive material must be stored and transported in containers that not only provide adequate interior capacity and structural integrity, but also shield those persons who handle the containers from the radioactivity of the material contained. For the type of radioactive material that is known as spent nuclear fuel, considerable shielding is necessary.
Spent nuclear fuel, and certain other types of radioactive waste, emits a great deal of gamma radiation and neutrons. Shielding gamma radiation is accomplished by making the container or spent fuel cask sufficiently massive. Higher density materials allow thinner-walled casks; lower density materials lead to thicker-walled casks.
Spent nuclear fuel also emits neutrons which are a form of radiation. Neutrons can be shielded best with certain kinds of material, notably those materials with a large number of hydrogen atoms.
In addition to capacity, structural integrity, and shielding, spent nuclear fuel casks must allow for the dissipation of heat energy from the radioactive decay taking place in the fuel. This decay heat can cause internal stresses when dissimilar materials are heated or when too great a heat gradient is established across a material used in cask construction.
There are two other considerations that limit the cask designer: weight and cost of the cask. For a cask that travels over the road, state highway restrictions disfavor vehicles weighing more than 80,000 pounds. Furthermore, greater cask weight can make loading and unloading more difficult.
The cost of materials and fabrication is a significant component of the costs of spent nuclear fuel storage and transportation. Another component is the number of casks needed for a given quantity of fuel. If cask payloads are small, more casks are needed to store and transport that quantity of spent nuclear fuel.
Thus, the cask designer is faced with competing choices of shape and materials for strength and shielding both gamma radiation and neutrons, while keeping weight and cost as low as possible.
Casks of various configurations have been constructed in the prior art to serve as containers for the transportation and storage of radioactive materials. Typically, they are cylindrical or square containers of cement or a dense metal such as steel or cast iron, having an inner cavity in which radioactive material being transported or stored is placed. These containers have heavy lids and trunnions to which lifting cables may be attached. The cask bodies are frequently composed of concentric layers of materials that provide structural strength and shielding. Until the present invention, the prior art has not successfully made any breakthrough in increasing payload without corresponding increases in weight. As the preferred embodiment of the present invention is the novel deployment of depleted uranium as shielding material in the form of pipes having polyethylene cores or in the form of rods, dispersed in a highly flexible pattern of bore holes around the periphery of a ductile iron cask, the discussion of the relevant art will be primarily directed to the prior use of uranium as a shielding material. Tungsten or other dense metal rods, as well as polyethylene rods, are viewed as alternative or additional shielding materials.
The use of shielding of multilayer construction was first noted in the radiation shield of Zinn in 1955 (U.S. Pat. No. 2,716,705). His shield comprised one or more layers of a neutron absorbing or shielding material and one or more layers of a gamma and neutron shielding material. Concrete, paraffin and steel shot were his protective materials. The Pat. No. of Trudeau et al. of 1973 (U.S. Pat. No. 3,732,427) illustrates an integrated transport system of that day. He mentions in his disclosure that the shielding used at the time was suitable only to attenuate gamma radiation and not the then recently discovered fast neutrons. Also, he mentions the use of lead, or lead and depleted uranium combined, as shielding materials. His claimed cask had an intermediate box-like barrier of depleted uranium which was 11/2inches thick. Also, his container had a barrier of hydrogenous material, wet plaster, to attenuate neutrons.
Hall's patent of 1978 (U.S. Pat. No. 4,123,392) discloses the method and use of non-combustible material, e.g. cement, containing various hydrogenous materials, such as plastic and resins, as shielding against neutrons. Reese's Pat. of 1979 (U.S. Pat. No. Re. 29,876) discloses an improved method of dissipating heat by fins externally located on the container. He mentions the possible use of depleted uranium as a beta radiation/gamma radiation absorbing material disposed between a central cavity for holding the radioactive material and an outer wall of corrosion resistant material. Heckman et al. in 1979 (U.S. Pat. No. 4,147,938) disclosed a system of bimetallic bands which expand when exposed to external heat to prevent fire damage to the inner portion of the cask. He mentions depleted uranium as material suitable for gamma shielding and hydrogenous materials for neutron shielding.
Baatz et al. in 1981 (4,272,683) discloses a transportation and storage vessel in which the intermediate layer of the body is a cast metal matrix in which uranium balls are embedded and the outer layer has channels containing a neutron absorber such as boron carbide and/or a moderator such as paraffin. A companion patent of Baatz et al., also in 1981 (U.S. Pat. No. 4,288,698), discloses a cask which contemplates a matrix of cast metal in which gamma-radiation absorbers are embedded. Also, he introduces the use of bore holes to hold neutron moderator material, in this case, water.
Christ et al. in 1984 (U.S. Pat. No. 4,451,739) discloses shielding for gamma radiation consisting of wire wrapped around the storage portion of the container. A jacket made of uranium, lead or steel is mentioned as a gamma radiation shield. Also, hollow spaces are disclosed between the layers of metal to be filled with neutron shielding material. A second Christ et al. patent, also in 1984 (4,453,081), emphasizes the use of graphite and boron carbide as neutron shielding materials.
Prior art known to this inventor includes the following U.S. Pat. Nos:
______________________________________ 2,716,705 8/1955 Zinn 3,732,427 5/1973 Trudeau et al. 4,123,392 11/1978 Hall et al. Re. 29,876 1/1979 Reese 4,147,938 4/1979 Heckman et al. 4,272,683 6/1981 Baatz et al. 4,288,698 9/1981 Baatz et al. 4,451,739 5/1984 Christ et al. 4,453,081 6/1984 Christ et al. ______________________________________